Conventionally, anti-corrosive paints containing an anti-corrosive pigment have been used for the corrosion prevention of a base material such as made of iron.
The Japanese Industrial Standards (JIS) specify eight kinds of anti-corrosive paints which differ depending on the kind of anti-corrosive pigments contained therein. Among those, six kinds of the anti-corrosive paints contain chromium-based or lead-based anti-corrosive pigments. The chromium-based anti-corrosive pigments specified by JIS include zinc chromate (ZPC type and ZTO type) and the like, and the lead-based anti-corrosive pigments include minium, lead suboxide, basic lead chromate and the like.
The above anti-corrosive pigments have superior anti-corrosive property and, therefore, have been widely used. However, they contain either chromium or lead which is harmful if absorbed into human bodies. Recently, there has been a growing demand for a pollution-free anti-corrosive pigment free from such harmful substances.
Examples of the conventionally known anti-corrosive pigments which contain neither chromium nor lead include phosphate-based pigments such as zinc phosphate and aluminum phosphate, and molybdate-based pigments such as zinc molybdate and calcium molybdate. However, any of the anti-corrosive pigments having superior anti-corrosive property comparable to that of the chromium-based or lead-based anti-corrosive pigments is yet to come in practice.
A barium metaborate, one of borate-based pigments, is also known as a low-pollution anti-corrosive pigment which contains neither chromium nor lead.
However, barium metaborate is specified to be a non-medical deleterious substance by the Japanese poisonous and Deleterious Substances Control Law and other applicable laws. Therefore, special attention should be given to the health and safety of a person who handles barium metaborate, and care should be taken not to allow people to directly touch a portion on which a paint containing the barium metaborate pigment is applied. Thus, the use of barium metaborate is limited by such situations.
Further, conventional anti-corrosive pigments such as zinc chromate are colored per se, thereby limiting the coloration of paints.
Inventors of the present invention have conducted several researches into a novel anti-corrosive pigment as an alternative to the conventional ones.
As a result, the inventors have had the findings that a strontium borate pigment represented by the general formula (1) is pollution-free and less toxic, and that it is excellent in anti-corrosive property and has no possibility of limiting the coloration of paints because it is white. EQU .alpha.SrO..beta.B.sub.2 O.sub.3..gamma.H.sub.2 O (1)
(wherein O&lt;.alpha..ltoreq.3, O&lt;.beta..ltoreq.4, O.ltoreq..gamma..ltoreq.5)
The inventors have also found a fact that the above strontium borate pigment is excellent not only in anti-corrosive property but also in anti-bacterial/anti-fungal and non-flammable properties.
However, it has been revealed that the anti-corrosive, anti-bacterial/anti-fungal and non-flammable properties cannot be maintained over a long period because the strontium borate pigment is water-soluble. Where the strontium borate pigment is dispersed in a material such as a paint or a molded product, penetration of moisture into the material causes the strontium borate pigment to dissolve and leach from the material or causes a paint coating film to be swelled and lifted, making it impossible to maintain the anti-corrosive, anti-bacterial/anti-fungal and non-flammable properties over an extended period.
The strontium borate pigment is typically prepared by reacting a water-soluble strontium salt with boric acid or a water-soluble borate in water. The strontium borate pigment thus prepared has large particle diameters, typically greater than about 7 .mu.m and, therefore, exhibits a reduced dispersibility in the material. This results in unsatisfactory anti-corrosive, anti-bacterial/anti-fungal and non-flammable properties.
It is an object of the present invention to provide a novel strontium borate pigment composition which has an excellent dispersibility in a material and an excellent water resistance and thereby allows the material to maintain superior anti-corrosive, anti-bacterial/anti-fungal and non-flammable properties over an extended period, an efficient method of making the same, and processes of imparting anti-corrosive, anti-bacterial/anti-fungal and non-flammable properties to the material by using the strontium borate pigment composition.